An example of a mechanical, cam driven pressurized fuel injector is provided in U.S. Pat. No. 4,467,963 to Sisson et al. on Aug. 28, 1984. It has a pumping piston biased upward by a return spring and operably displaced downward by a cam driven rocker arm. A solenoid operated control valve, disposed between a pressure chamber on which the piston acts, and a low pressure fuel pump, is movable between an open position and a closed position. With the solenoid in the closed position, fuel in a timing chamber pressurized by the downwardly moving pumping piston is blocked from reaching the fuel pump and is forced against a metering piston. The metering piston pressurizes fuel in a metering chamber, forcing it through flow orifices, thereby injecting it into the combustion chamber. The solenoid moves to the open position when the piston is moving upward, allowing the timing chamber to be replenished by the fuel pump.
Present and future emission laws, and performance requirements, require that today's fuel systems pressurize fuel to pressures on the order of 138 MPa (20,000 psi) or higher. The entire mechanism, including solenoid operated valves, such as the one discussed above, must sustain fuel pressures of this magnitude. Such pressures can be difficult to sustain over extended periods of time without developing leaks between the pumping piston and the surrounding housing. Additionally, solenoid operated valves in injectors like that disclosed by Sisson et al. must be fabricated with a high degree of precision to prevent leakage at the valve seats under maximum pressure conditions.
U.S. Pat. No. 5,121,730 to Ausman et al. on Jun. 16, 1992, discloses a hydraulically actuated, electronically controlled injector which reduces the risk of fuel leakage by elevating the pressure of the fuel at a location proximate to the injection end of the injector. In this injector, moderately pressurized hydraulic fluid, for example, engine lubrication oil at about 23 MPa (3,335 psi), pressurized by an external pump, is used to pressurize fuel through an intensifier piston which multiplies the pressure of the oil to pressurize the fuel at a location relatively close to the injection orifices. The resultant fuel pressure is approximately 161 MPa (23,345 psi). This system, however, requires providing the external oil pump which elevates the oil pressure to the 23 MPa level.
It is desired to provide a fuel injector employing pressurized hydraulic fluid in combination with an intensifier piston to pressurize the fuel without employing an external pump to increase the pressure of the hydraulic fluid. It is also desired to minimize any cold start problems associated with the use of higher viscosity hydraulic fluid in the injectors.
The present invention is directed to overcoming one or more of the problems as set forth above.